1. Field of the Invention
The present invention relates generally to the field of geothermal steam production and more particularly to methods for capping-off completed, but unused, geothermal steam wells.
2. Discussion of the Prior Art
Large subterranean reservoirs of naturally-occurring steam and/or hot aqueous liquid can be found in many regions of the world. Such reservoirs of geothermal steam and water or brine are particularly prevalent in regions where the thermal gradient near the earth's surface is abnormally high, as in regions of volcanic, geyser and fumarole activity, for example, along the rim of the Pacific Ocean.
In some areas, where readily available and conveniently located, geothermal steam and water or brine have for some time been advantageously used for therapeutic purposes, industrial processes and direct heating. Although there is current interest in further developing goethermal resources for these purposes, principal effort has more recently been directed more towards developing geothermal resources for production of electric power, the use of which is far less site-restricted than is the more direct use of the geothermal fluids for the above-mentioned purposes. In particular, the relatively recent, steep increases in hydrocarbon fuel costs and actual or threatened shortages of heretofore cheap and abundant hydrocarbon fuels has greatly heightened interest in development of alternative fuel sources, including the use of geothermal fluids for electric power generation.
General techniques are known whereby hot geothermal fluids can be used to generate electric power. For example, geothermal steam can be used in substantially the same manner as boiler-generated steam to drive a steam turbine/electric generator combination. Pressurized geothermal water or brine, having a temperature above about 400.degree. F., can be flashed to a lower pressure to extract steam which is then used to drive a steam turbine/generator. Lower temperature geothermal water or brine can be used in a binary system to vaporize, in a closed loop, a working fluid, the resulting vapor being used to drive a gas turbine/generator.
Use of geothermal steam for production of electric power is the most direct geothermal application and is therefore preferred, being generally easier and less costly to use than geothermal water or brine for power generation. Consequently, although commercially usable sources of geothermal steam are estimated to be only about one fifth as prevalent as those of geothermal water or brine, considerable effort has been, and is being, directed towards developing new, or expanding existing, geothermal steam power plants. For example, since development of geothermal steam power plants started at The Geysers in California in the 1960's, electric power production by geothermal steam has there increased to a current level of about 1,000 megawatts, with development continuing towards a currently estimated capacity of about 1,500 to 2,000 megawatts. As a point of reference, an estimated five percent of the electric power generated in California is now being geothermally generated at The Geysers.
Continued development of geothermal steam for electric power production, in such locations as The Geysers, requires the building of new power plants and the annual drilling of many geothermal steam wells for providing steam to these new power plants.
By way of illustration, since about 20 pounds of geothermal steam is, on the average, required for each kilowatt of electric power produced, a typical 100 megawatt geothermal steam power plant requires about two million pounds of geothermal steam per hour. As good geothermal steam wells usually produce between about 150,000 and 200,000 pounds of steam per hour, each such typical geothermal steam power plant requires between about 10 and 15 geothermal steam wells for supplying steam.
Most geothermal steam wells require extensive drilling times and relatively high costs before they can be put into production. The high well drilling cost and comparatively long drilling time reflect the severe problems often encountered in drilling geothermal steam wells. The problems include the penetration of difficult geological formations, high well temperatures (typically about or above 500.degree. F.), corrosive and abrasive characteristics of the air drilling process normally used in combination with the hot steam encountered, and the frequently remote and poorly accessible drill site locations.
Because of the 10 to 15 geothermal steam wells typically required for each new geothermal steam power plant and the high drilling costs and long drilling times involved, the drilling operations are usually spread over several years, for example, over the 3-to-5-year construction time of the related power plant. Although a protracted well drilling operation of this nature is advantageous from standpoints of capital outlay and optimum drilling equipment utilization, problems relating to completed geothermal steam wells standing idle for long periods of time, typically at least about a year and sometimes as long as four years, are thereby created. In particular, these problems relate to keeping the wells in operational condition without substantial steam loss or violation of air pollution standards that arise from inherent geothermal steam characteristics.
In this regard, the bottom 2,000 to 3,000 feet of most geothermal steam wells in the steam-producing zone are ordinarily uncased to enable the necessary high steam extraction rates. When geothermal steam wells of this type are shut in after completion and before use, so as to conserve steam and prevent air pollution, steam entering the lower, uncased well region from the surrounding formation rises in the borehole and condenses in cooler, upper borehole regions. As the resulting condensate flows back down the borehole, rocks and other debris along the uncased well region are loosened and washed down into the bottom, steam production zone. These fallen rocks and debris, as well as the condensate itself, soon fill the steam-producing zone and "kill" the well. Before being later operatively connected to a power plant, the well requires rework with a drilling rig, and to avoid the high costs associated with steam well reconditioning, most completed, but idle, geothermal steam wells have heretofore continuously vented an amount of geothermal steam sufficient to prevent well damage by steam condensation in the well. That is, sufficient steam has been vented from the wellhead of idle steam wells to maintain the temperature throughout the well above the steam condensation point. The amount of geothermal steam required to be vented for this purpose, of course, varies from well to well and according to the quality of the steam, but has been found to be typically between about 200 and 30,000 pounds per hour.
Venting of steam from geothermal wells to prevent condensation damage, although usually satisfactory for its intended purpose, not only wastes steam but, more importantly, causes air pollution problems which in many areas threaten its continued practice. Hydrogen sulfide is virtually always present in geothermal steam due, at least in part, it is believed, to action of anerobic bacteria on sulfides naturally present in the ground. The hydrogen sulfide concentration of the vented geothermal steam is typically in a range of between about 40 parts per million (ppm) and 1,000 ppm, which is usually higher than the point source hydrogen sulfide emission standards of between about 1 pound per hour per vent and 4.4 pounds per hour per vent applicable in many locations.
Although such strict hydrogen sulfide emission standards have not been uniformly enforced in the past, as the number of geothermal steam wells drilled increases and their intrusion into populated and/or environmentally protective localities grows, more rigorous enforcement of these emission standards is virtually certain. The expected result is that venting of geothermal steam wells to prevent condensation damage may soon be prohibited in many areas unless costly hydrogen sulfide abatement processes are provided.
Similar strict hydrogen sulfide emission standards are also usually applied to "used" steam discharged into the atmosphere from operational geothermal power plants and to the large scale venting, or "stacking," of geothermal steam during brief periods of power plant shutdown or slowdown. However, because of the large amounts of steam and hydrogen sulfide involved and the high cost of the power plant, expensive and complex hydrogen sulfide removal facilities of a permanent nature are feasible and are normally provided.
Unfortunately, facilities of the type used for treating large volumes of steam discharged from geothermal steam power plants, and which may, for example, utilize hydrogen sulfide removal processes, such as disclosed in U.S. Pat. No. 4,283,379 to Fenton et al., are not economically adaptable to removing hydrogen sulfide from the relatively much smaller quantities of steam vented in numerous, isolated locations from idle steam wells to prevent condensation damage.
The strict emission standards are usually also applied to hydrogen sulfide emissions in escaping drilling gas and steam during actual geothermal steam well drilling operations. Because processes and apparatus used for power plant hydrogen sulfide abatement have also not been found economically adaptable for well drilling operations, other hydrogen sulfide abatement processes have been developed for this purpose. One such hydrogen sulfide abatement process particularly useful for geothermal steam well drilling operations is disclosed in U.S. Pat. No. 4,151,760 to Woertz. Although the process disclosed by Woertz has been determined to be effective for removing hydrogen sulfide from emissions during steam well drilling operations and to be comparatively economical for this purpose, it is not as economically attractive for abating hydrogen sulfide emissions from vented, idle steam wells.
Accordingly, it is an object of the present invention to provide an effective, comparatively inexpensive method for preserving the operations integrity of idle geothermal steam wells which also satisfies strict hydrogen sulfide emission standards.
Another object of the present invention is to provide a method for capping, for long periods of time, an idle geothermal steam well without allowing substantial geothermal steam condensation and well damage due to such condensation.
Additional objects, advantages and features of the invention will become apparent to those skilled in the art from the following description.